Means to promote the commutation of a thyristor switching arrangement for an electric motor at low motor speeds

ABSTRACT

An electric motor system consisting of switching means, such as an inverter, and a synchronous motor, means to promote the commutation action of the switching means at starting, comprising two polyphase windings mounted on the stator of the synchronous motor with magnetic coupling, one of the polyphase windings being connected to produce the rotating magnetic field with the direct current source through the switching means and the other polyphase winding being energized by an external alternating current source in order to supply commutating energy to the switching means.

0 United States Patent [151 3,660,739

Okuyama et al. 5] May 2, 1972 [54] MEANS TO PROMOTE THE [56] ReferencesCited COMMUTATION OF A THYRISTOR SWITCHING ARRANGEMENT FOR AN UN'TEDSTATES PATENTS ELECTRIC MOTOR AT ow MOTOR 3,262,036 7/l966 Clarke et a]..3l8/l 38 x SPEEDS Primary Examiner-Gene Z. Rubmson [72] Inventors:Toshiaki Okuyamu; Hiroshi Watanabe, Attorney-Craig. Antonelli. Stewart&Hill both of Hitachi Japan (73] Assignee: Hitachi, Ltd., Tokyo. Japan[57] ABSTRACT [22] Filed1 net 8, 1970 An electric motor systemconsisting of switching means. such as an inverter, and a synchronousmotor, means to promote pp NOJ 96,169 the commutation action of theswitching means at starting comprising two polyphase windings mounted onthe stator of [30] Foreign Application Priority Dat he synchronous motorwith magnetic coupling, one of the polyphase windings being connected toproduce the rotating D i969 Japan 2 magnetic field with the directcurrent source through the switching means and the other polyphasewinding being ener- U5. Cl 313/227, 318/230 gized by an externalalternating current source in order to 1 CL 5/40 supply commutatingenergy to the switching means. [58] FieldofSearch..318/l7l,138,227,230,254

12 Claims, 10 Drawing Figures [fiDZl D12 Z QFD 1 0.1 E 03.

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' AND-GATE AND-GATE Famine 6 All A3l Am 1 GATE AMPL GATE ANPL GATE ANPLAZIT A32 A22 GATE AMP'L GATE AWL GATE AMPL Su S2: S2 S32 SIZ S22Patented May 2, 1972 3,660,739

4 Sheets-Sheet I5 FIG 4a 4 L fin l 2 G31, 3 GlZ 5 I o AND-GATE AND-GATEAND-GATE 3 G2!) (532 G22, 5 AND-GATE AND-GATE AND-GATE C 6 An Aan AmGATE AMPL GATE ANPL GATE AMPL AZIR A32 A22 GATE AMPL GATE ANPL GATEAMPLSn SZI Sal S32 S|2 S22 INVENTORS Tosmmu ouuxnmq 2r maosm vim'nu naz M MJ" saw k \10 u qwom avg BACKGROUND OF THE INVENTION The present inventionrelates to improvements in electric motor control systems including theuse of thyristors.

A motor control system using thyristors is well known as a means ofcontrolling a variable speed motor (e.g., direct current motor), inwhich contactless switching elements, such as gate turn on and nongatetum-off controlled rectifiers, are used instead of the commutator andthe brush to eliminate the malfunctions caused by friction. These kindsof electric motor control systems are called thyristor motor systems, orsometimes brushless motor systems.

Generally, the thyristor motor system consists of an alternating currentmotor and switching means, such as an inverter or converter, and isclassified into an a.c. input type and a dc. input type by the sort ofinput voltage applied to said switching means. Both types of systemshave respective features. As the present invention particularly relatesto the d.c. input type, the following description relates particularlyto that type.

The dc input type thyristor motor system comprises an alternatingcurrent motor and switching means for converting a direct current to apolyphase alternating current. The switching means comprise a pluralityof semiconductor controlled rectifiers, so-called thyristors. Thethyristors are turned on by gate signals applied to the gate electrodesone after another in a predetermined order according to the relativeposition of the stator and the rotor of the alternating current motor,and are commutated by the backwardly applied counter electromotive forceof the alternating current motor.

It is well known that the counter electromotive force of an alternatingcurrent motor is proportional to the rotating speed of the motor, if thefield intensity is constant. Therefore, when the speed of the motor isvery low, the voltage does not become high enough to commutate thethyristors. Most notably, the counter electromotive force is very low ator during starting.

When a larger starting torque is required, the above-mentionedcommutation becomes more difficult. Accordingly, a conventional d.c.input type thyristor motor system normally uses a starting means, whichis mechanically linked with the rotor of the motor and rotates the rotorat starting in order to produce the necessary counter electromotiveforce. The starting means not only complicates the structure of thethyristor motor system but makes it very expensive.

SUMMARY OF THE INVENTION An object of the present invention is toprovide an electric motor system controlled by thyristors which isdesigned so as to easily perform the required commutation at or duringstarting or at a low motor speed.

Another object of the present invention is to provide a thyristor motorsystem which can start with a heavy load.

According to one aspect of the present invention, an electric motorsystem comprises a rotating machine having a polyphase windingcomprising a plurality of windings in either a stator or a rotor,switching means for connecting the individual windings to a directcurrent source one after another in a predetermined order according tothe relative position of the stator and the rotor of the rotatingmachine, and additional commutating means for supplying the commutationenergy to the switching means from the external source through thepolyphase winding in order to promote the commutation action of theswitching means.

Other objects and features of the present invention will become apparentfrom the following description of embodiments of the invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an electrical circuitdiagram showing a preferred embodiment of the invention;

FIGS. 20 and 2b, and FIGS. 30, 3b and 3c are waveform and other diagramsfor explaining the action of the system shown in FIG. 1;

FIGS. 40 and 4b are schematic circuit diagrams showing a control circuitfor the system shown in FIG. I; and

FIG. 5 and FIG. 6 are circuit diagrams, respectively, showing differentpreferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One of the preferredembodiments of the present invention will be described with reference toFIG. 1. The rotating machine used in this embodiment is a three-phasealternating current motor and consists of a rotor (not shown) and astator having additional three-phase windings (Us, Vs, Ws) mounted on astator core together with stationary windings (Up, Vp, Wp). The twopolyphase windings are magnetically coupled on the same stator core. Theadditional three-phase winding is connected with an external three-phasealternating current source through a switch SW at terminals r, .r and t.The stationary winding is connected with the output terminals x, y and zof a thyristor switching means.

The switching means comprises six gate turn-on, nongate tum-offcontrolled rectifiers, namely thyristors S,,, S,,, S S S and 8;,connected in three phase graetz connection. Input terminals) and m aresupplied with direct current voltage and the thyristors are ignited oneafter another in a predetermined order according to the relativeposition of the stator and the rotor of the rotating machine, whereby athreephase alternating current voltage is induced at the outputterminals x, y and z.

The three-phase stationary winding energized by the voltage inducedacross the terminals 1:, y and z produces the threephase rotatingmagnetic field and the rotor is caused to rotate by such a rotatingmagnetic field. While the rotor is rotating, the counter electromotiveforces are produced in the stationary winding (Up, Vp, Wp).

FIGS. 20 and 2b, respectively, show the change of the abovementionedcounter electromotive forces and the ignitiori order of the thyristors.Referring to these figures, the rotating action of an ordinary thyristormotor system will be described hereunder. The curve U shows a voltageproduced on a winding Up, and similarly curves V and W, respectively,show voltages produced on windings Vp and Wp.

The points r,-t, are time points at which the commutation is carried outfrom the one thyristor to the other. In FIG. 2b, each duration shown bythe symbol corresponding to each thyristor represents the conductingperiod of the thyristor.

For example, during the time between points I, and t the thyristors 5,,and 5;, are conductive and windings Up and Wp are connected with thedirect current source. Then, the winding Up and Wp induce the voltagesas shown by curves U and W in FIGS. 20. As the winding Vp induces thevoltage shown by curve V, the electrical potential of the anode of thethyristor 5,, equals the voltage (V W), measured from a negativeterminal m. Further the electrical potential of the cathode thereofbecomes voltage (U W) in like manner. Therefore, the voltage difference(U V) is applied forwardly across the anode and the cathode of thethyristor S,,.

At the time point t;,, the thyristor 5,, is ignited and becomesconductive so that the voltage difference (U V) is applied backwardly tothe thyristor 5,, through the circuit of 5,, Vp Up S,, 5 As well known,in order to recover the cut-off condition of a thyristor in conduction,the backward voltage must be applied to the thyristor across the anodeand the cathode thereof, or the forward current through the thyristormust decrease down to less than the conduction holding current. In saidcase, the voltage (U V) is used as the abovementioned backward voltage.After the thyristor S,, is turned off thereby, windings Vp and Wp areconnected to the direct current source through the thyristors S and S Asabove-mentioned, if the rotor rotates at a high speed, the commutationof each thyristor is very easily performed, since each winding producesa voltage high enough to commutate the thyristors. But, if the speed ofthe rotor is extremely low or zero, the thyristors cannot commutate bymeans of the counter electromotive force.

The present invention provides an additional commutating means forpromoting turn-off of the thyristors while the speed of the rotor isextremely low or zero. In FIG. la, the additional windings Us, Vs and Wsare provided on the stator core, which windings are magnetically coupledwith windings Up, Vp and Wp. If the alternating current voltage isapplied to the windings Us, Vs and Ws, alternating current voltages willbe induced in the windings Up, Vp and Wp by transformer action. Thevoltage applied to terminals r, s and t, and the turn ratio betweenadditional winding (Us, Vs, Ws) and stationary winding (Up, Vp, Wp) willbe determined such that the voltages induced in the windings Up, Vp andWp become high enough to commutate the thyristors.

FIGS. 3a, 3b and 3c explain the function of the system when the rotor isrotated at a very low speed. FIG. 3a shows the counter electromotiveforces, FIG. 3b shows the three-phase voltages U, V and W applied acrossthe windings Us, Vs and Ws, and FIG. 3c shows the conducting term ofeach thyristor. The symbols used in FIGS. 3a, 3b and 3c are similar tothose in FIGS. 2a and 2b except for voltages U, V and W'.

While the speed of the rotor is low, the counter electromotive forcesare extremely low, as shown in FIG. 30. At that time, the switch SW isswitched on. The windings Us, Vs and Ws thereby receive the voltages asshown in FIG. 3b. The voltages applied across the windings Us, Vs and Wsinduce in the windings Up, Vp and Wp the predetermined voltages havingthe same waveform as shown in FIG. 3b.

Then, if the gate signal is fed to the gate of the thyristor S at thetime point the thyristor 3,, turns on, but yet the thyristor 5,, cannotturn off, because the counter electromotive force is extremely low asshown at a'-b in FIG. 3a, and further the voltage (U V) is negative asshown in FIG. 3b so that a forward voltage is applied to the thyristor5,, as a whole.

At the time point r',,, however, the voltage (U V') becomes positive.Hereafter, to the thyristor 8,, is applied a backward voltage highenough to commutate. Accordingly, the overlapping interval A (t', toccurs, and in the interval A both thyristors S,, and 8,, areconductive. After the time interval A, the thyristor 8,, turns off andonly the thyristor S. is conductive.

If the frequency of the voltage applied across windings Us, Vs and Ws is50 or 60 Hertz per second (commercial frequency in Japan), the voltage(U V) being positive is sure to exist during an angle yo. Thus, eachthyristor is ignited at a predetermined electric angle 70 after avoltage of one phase is equal to that of the other. The electric angleis generally called the control angle, and as well known, this isdetermined by a margin angle for commutation and an overlapping angle.The former is determined by a turn-off time of the thyristor used and/ora ripple rate of a current flowing through the thyristor, and the latteris influenced by the commutation inductance of the circuit and the valueof the load current.

On the other hand, the frequency of the voltage shown by U', V or W ismuch higher than that of the voltage shown by U, V or W. If the controlangle 70 is selected to be sufficiently large, the voltage (U' V) beingpositive is sure to exist during the control angle yo. The rotoracceleration and the counter electromotive force having beensufficiently high, the switch SW is opened. Hereafter, all thyristorscan be commutated by electromotive force.

As an example, the switch SW may be opened when the counterelectromotive force has become about I0 percent of the voltagecorresponding to the rated speed. As mentioned above, the frequency andthe value of the voltage applied across windings Us, Vs and W shall beselected such that a sufficiently high voltage is applied to thethyristors as a backward voltage when the switch SW is closed.

Next, referring to FIGS. 4a and 4b, the control circuit of thethyristors as shown in FIG. I is explained. In these figures, the

symbols D I) D D D, and D designate the position detectors. SEdesignates the segment, which is linked with the rotor and given therotation in the direction shown by the arrow. The position detectorfaced by the segment produces the output which represents the positionof the rotor. Each output is applied to AND-gates G G,,, G,,, G G and0;, respectively. The AND-gates are opened by signals applied toterminals l to 6. The outputs of the AND-gates are suppled to the gatesof the thyristors through gate amplifiers A A A A", A and A33.

The signals applied to terminals 1 to 6 are produced from Flip-Flopcircuits FF,, FF, and FF, shown in FIG. 4b. These Flip-Flop circuits arealternately and repeatedly set or reset by the polarities ofthe voltages(U V), (V W) and (W U). For example, Flip-Flop FF, is reset to producean output on the terminal 2 while the voltage (U V) is positive, i.e.,the duration shown by hatching in FIG. 3b. The output appearing on theterminal 2 is applied to the AND-gate G and the AND-gate G is openedthereby. At that time, if the position detector D produces an output,the output is applied to the gate of the thyristor 8,, through the gateamplifier A The F lip-Flops FF, and F F, operate also in like manner.

Specific means for alternately setting or resetting these F lip- Flopsis not shown in the figure; however, such means are easily provided. Forexample, the three voltage detectors are provided and they detect eachphase voltage U, V and W applied to the windings Us, Vs and Ws (thesevoltages U, V and W synchronize with the voltages U, V and W induced inthe windings Up, Vp and Wp). Further, voltages (U' V), (V' W) and (W U)are derived by the detected voltage U, V and W, and if the voltage (U V)is negative, Flip-Flop FF is set, if the voltage (V' W) is negative,Flip-flop FF,, is set, and if the voltage (W'U) is negative, Flip-FlopFF, is set.

But, if the two thyristors are permitted to conduct simultaneouslyduring commutating from one of the thyristors to the other, for example,as shown in FIG. 3c, the above-mentioned Flip-Flops are not alwaysnecessary.

As mentioned above, in the circuit shown in FIG. I, the energy forcommutating the thyristors is provided by an external source through theadditional commutating means. Accordingly, the thyristors in thisembodiment are easily commutated even at a low speed of rotation.

Next, referring to FIGS. 5 and 6, where similar reference symbols areused to denote the same parts as shown in FIG. 1, T,, T, and T,represent transformers, which provide the energy of commutation to thethyristors (not shown). The transfer mers are modified three-phasetransformers, of which the primary winding has a star-connection and isconnected with terminals r, .r and 1 through a switch SW. The one end ofeach secondary winding is connected with windings U, V or W,respectively, and the other end is connected to terminals x, y or 2,respectively. The thyristor portion in FIG. 5 is similar to that in FIG.I. 5,, S, and 8,, represent switches provided to short-circuit thesecondary winding of the transformer.

In this case, when the speed of the motor exceeds the predeterminedspeed, the switch SW is opened and switches 5,, S, and S, are closed sothat the transformer is short-circuited.

Further, as shown in FIG. 6, the neutral point of the polyphase windingsU, V and W is separated, and polyphase transformers T,, T, and T,, maybe inserted therein. In the transformers of this embodiment, both theprimary and the secondary windings are star-connected, and the primarywindings are connected to terminals r, s and I through a switch SW inthe same manner as the embodiment of FIG. 5. The secondary windings areconnected with the separated neutral points Nu, Nv and Nw, and switchesS, and S, are connected respectively in parallel with the two out ofthree secondary windings so that all of the windings are short-circuitedby closing the switches S, and S The operation of dais circuit isentirely similar to that of the circuit shown in FIG. 5.

While we have shown and described several embodiments in accordance withthe present invention, it is understood that the same is not limitedthereto but is susceptible of numerous changes and modifications asknown to a person skilled in the art, and we therefore do not wish to belimited to the details shown and described herein but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

We claim:

1. An electric motor system comprising a direct current source; rotatingmachine means including a polyphase winding for producing a rotatingmagnetic field in either a stator or a rotor; switching means includinga plurality of gate turn-on, nongate turn-off controlled rectifiers forconnecting the individual windings on the polyphase winding to thedirect current source by rendering the controlled rectifiers conductiveone after another in a predetermined order according to the relativeposition of the stator and the rotor of the rotating machine; additionalcommutating means including an additional polyphase winding associatedwith said polyphase winding for promoting the turn-off of the controlledrectifiers by absorbing reactive power stored in said rotating machinemeans and applying a backward voltage to said controlled rectifiers tobe turned off through said polyphase windings; and a polyphasealternating current source, having a number of phases equal to thenumber of polyphase windings, for supplying alternating current power tothe additional commutating means.

2. An electric motor system comprising a direct current source, rotatingmachine means including a polyphase winding for producing a rotatingmagnetic field in either a stator or a rotor; switching means includinga plurality of gate turn-on, nongate turn-off controlled rectifiers forconnecting the individual windings of the polyphase winding to thedirect current source by rendering the controlled rectifiers conductiveone after another in a predetermined order according to the relativeposition of the stator and the rotor of the rotating machine; additionalcommutating means associated with said polyphase winding for promotingthe tum-off of the controlled rectifiers by applying a backward voltageto said controlled rectifiers to be turned off through said polyphasewindings; and a polyphase alternating current source, having a number ofphases equal to the number of polyphase windings, for supplyingalternating current power to the additional commutating means, whereinsaid additional commutating means includes another polyphase windingwhich is magnetically coupled with the polyphase winding of saidrotating machine on an iron core thereof in order to induce the backwardvoltage in the polyphase winding of the rotating machine by transformeraction.

3. An electric motor system comprising a direct current source; rotatingmachine means including a polyphase winding for producing a rotatingmagnetic field in either a stator or a rotor; switching means includinga plurality of gate turn-on, nongate turn-off controlled rectifiers forconnecting the individual windings of the polyphase winding to thedirect current source by rendering the controlled rectifiers conductiveone after another in a predetermined order according to the relativeposition of the stator and the rotor of the rotating machine; additionalcommutating means including an additional polyphase winding associatedwith said polyphase winding for promoting the tum-off of the controlledrectifiers by absorbing reactive power stored in said rotating machinemeans and applying a backward voltage to said controlled rectifiers tobe turned off through said polyphase windings; and a polyphasealternating current source, having a number of phases equal to thenumber of polyphase windings, for supplying alternating current power tothe additional commutating means, wherein said additional commutatingmeans includes a transformer having primary windings connected to thealternating current source, secondary windings connected with thepolyphase windings and switch means for short-cin cuiting the secondarywindings.

4. An electric motor system according to claim 3, wherein said lyphasewinding is stanconnected, and said secondary win tngs are connected witheach phase of the polyphase winding, respectively.

5. An electric motor system according to claim 3, wherein said polyphasewinding is star-connected, the neutral point of which is opened by eachphase, and each said secondary winding is respectively connected withthe opened neutral points according to each phase.

6. An electric motor system comprising a direct current source; rotatingmachine means including a polyphase winding for producing a rotatingmagnetic field in either a stator of a rotor; switching means includinga plurality of gate turn-on, nongate turnoff controlled rectifiers forconnecting the individual windings of the polyphase winding to thedirect current source be rendering the controlled rectifiers conductiveone after another in a predetermined order according to the relativeposition of the stator and the rotor of the rotating machine; additionalcommutating means including an adtli tional polyphase winding associatedwith said polyphase winding for promoting the turn-off of the controlledrectifiers by absorbing reactive power stored in said rotating machinemeans and applying a backward voltage to said controlled rectifiers tobe turned off through said polyphase windings; and a polyphasealternating current source, having a number of phases equal to thenumber of polyphase windings, for supplying alternating current power tothe additional commutating means, wherein means is provided toelectrically disconnect said additional commutating means from saidpolyphase winding when said rotating machine starts up to a speedproducing a voltage high enough to independently commutate saidcontrolled rectifiers,

7. An electrical control arrangement for increasing the starting counterelectromotive force of a polyphase electric motor winding made up ofwinding portions selectively con nected to a source of direct current byswitching means including a plurality ofgate tum-on, nongate turn-offcontrolled rectifiers under the control of means rendering thecontrolled rectifiers conductive one after another in a predeterminedorder according to the relative position of the stator and the rotor ofthe motor, comprising an additional polyphase winding coupled with saidpolyphase electric motor winding, and a polyphase alternating currentsource, having a number of phases equal to the number of windingportions making up said polyphase electric motor winding, selectivelyconnected to said additional polyphase winding for supplying alternatingpower thereto.

8 An electrical control arrangement as defined in claim 7, wherein saidpolyphase electric motor winding and said additional polyphase windingare magnetically coupled by means of an iron core.

9. An electrical control arrangement as defined in claim 7, wherein saidadditional polyphase winding is made up of secondary windings of atransformer, said transformer having an equal number of primary windingsconnected to said alternating current source and additional switch meansfor selectively shortcircuiting said secondary windings,

10. An electrical control arrangement as defined in claim 9, whereinsaid polyphase winding is star-connected, and said secondary windingsare connected with each phase of the polyphase winding, respectively.

11. An electrical control arrangement as defined in claim 9, whereinsaid polyphase winding is star-connected, the neutral point of which isopened by each phase, and each said secondary winding is respectivelyconnected with the opened neutral points according to each phase.

12. An electrical control arrangement as defined in claim 7, whereinmeans is provided to electrically disconnect said additional polyphasewinding from said alternating current source when the motor reaches apredetermined speed after starting sufficient to produce a counterelectromotive force high enough to independently commutate saidcontrolled rectifiers.

1. An electric motor system comprising a direct current source; rotatingmachine means including a polyphase winding for producing a rotatingmagnetic field in either a stator or a rotor; switching means includinga plurality of gate turn-on, nongate turn-off controlled rectifiers forconnecting the individual windings on the polyphase winding to thedirect current source by rendering the cOntrolled rectifiers conductiveone after another in a predetermined order according to the relativeposition of the stator and the rotor of the rotating machine; additionalcommutating means including an additional polyphase winding associatedwith said polyphase winding for promoting the turn-off of the controlledrectifiers by absorbing reactive power stored in said rotating machinemeans and applying a backward voltage to said controlled rectifiers tobe turned off through said polyphase windings; and a polyphasealternating current source, having a number of phases equal to thenumber of polyphase windings, for supplying alternating current power tothe additional commutating means.
 2. An electric motor system comprisinga direct current source, rotating machine means including a polyphasewinding for producing a rotating magnetic field in either a stator or arotor; switching means including a plurality of gate turn-on, nongateturn-off controlled rectifiers for connecting the individual windings ofthe polyphase winding to the direct current source by rendering thecontrolled rectifiers conductive one after another in a predeterminedorder according to the relative position of the stator and the rotor ofthe rotating machine; additional commutating means associated with saidpolyphase winding for promoting the turn-off of the controlledrectifiers by applying a backward voltage to said controlled rectifiersto be turned off through said polyphase windings; and a polyphasealternating current source, having a number of phases equal to thenumber of polyphase windings, for supplying alternating current power tothe additional commutating means, wherein said additional commutatingmeans includes another polyphase winding which is magnetically coupledwith the polyphase winding of said rotating machine on an iron corethereof in order to induce the backward voltage in the polyphase windingof the rotating machine by transformer action.
 3. An electric motorsystem comprising a direct current source; rotating machine meansincluding a polyphase winding for producing a rotating magnetic field ineither a stator or a rotor; switching means including a plurality ofgate turn-on, nongate turn-off controlled rectifiers for connecting theindividual windings of the polyphase winding to the direct currentsource by rendering the controlled rectifiers conductive one afteranother in a predetermined order according to the relative position ofthe stator and the rotor of the rotating machine; additional commutatingmeans including an additional polyphase winding associated with saidpolyphase winding for promoting the turn-off of the controlledrectifiers by absorbing reactive power stored in said rotating machinemeans and applying a backward voltage to said controlled rectifiers tobe turned off through said polyphase windings; and a polyphasealternating current source, having a number of phases equal to thenumber of polyphase windings, for supplying alternating current power tothe additional commutating means, wherein said additional commutatingmeans includes a transformer having primary windings connected to thealternating current source, secondary windings connected with thepolyphase windings and switch means for short-circuiting the secondarywindings.
 4. An electric motor system according to claim 3, wherein saidpolyphase winding is star-connected, and said secondary windings areconnected with each phase of the polyphase winding, respectively.
 5. Anelectric motor system according to claim 3, wherein said polyphasewinding is star-connected, the neutral point of which is opened by eachphase, and each said secondary winding is respectively connected withthe opened neutral points according to each phase.
 6. An electric motorsystem comprising a direct current source; rotating machine meansincluding a polyphase winding for producing a rotating magnetic field ineither a stator of a rotor; switching means including a plurality ofgate turn-on, nongAte turn-off controlled rectifiers for connecting theindividual windings of the polyphase winding to the direct currentsource be rendering the controlled rectifiers conductive one afteranother in a predetermined order according to the relative position ofthe stator and the rotor of the rotating machine; additional commutatingmeans including an additional polyphase winding associated with saidpolyphase winding for promoting the turn-off of the controlledrectifiers by absorbing reactive power stored in said rotating machinemeans and applying a backward voltage to said controlled rectifiers tobe turned off through said polyphase windings; and a polyphasealternating current source, having a number of phases equal to thenumber of polyphase windings, for supplying alternating current power tothe additional commutating means, wherein means is provided toelectrically disconnect said additional commutating means from saidpolyphase winding when said rotating machine starts up to a speedproducing a voltage high enough to independently commutate saidcontrolled rectifiers.
 7. An electrical control arrangement forincreasing the starting counter electromotive force of a polyphaseelectric motor winding made up of winding portions selectively connectedto a source of direct current by switching means including a pluralityof gate turn-on, nongate turn-off controlled rectifiers under thecontrol of means rendering the controlled rectifiers conductive oneafter another in a predetermined order according to the relativeposition of the stator and the rotor of the motor, comprising anadditional polyphase winding coupled with said polyphase electric motorwinding, and a polyphase alternating current source, having a number ofphases equal to the number of winding portions making up said polyphaseelectric motor winding, selectively connected to said additionalpolyphase winding for supplying alternating power thereto.
 8. Anelectrical control arrangement as defined in claim 7, wherein saidpolyphase electric motor winding and said additional polyphase windingare magnetically coupled by means of an iron core.
 9. An electricalcontrol arrangement as defined in claim 7, wherein said additionalpolyphase winding is made up of secondary windings of a transformer,said transformer having an equal number of primary windings connected tosaid alternating current source and additional switch means forselectively short-circuiting said secondary windings.
 10. An electricalcontrol arrangement as defined in claim 9, wherein said polyphasewinding is star-connected, and said secondary windings are connectedwith each phase of the polyphase winding, respectively.
 11. Anelectrical control arrangement as defined in claim 9, wherein saidpolyphase winding is star-connected, the neutral point of which isopened by each phase, and each said secondary winding is respectivelyconnected with the opened neutral points according to each phase.
 12. Anelectrical control arrangement as defined in claim 7, wherein means isprovided to electrically disconnect said additional polyphase windingfrom said alternating current source when the motor reaches apredetermined speed after starting sufficient to produce a counterelectromotive force high enough to independently commutate saidcontrolled rectifiers.